1. Field of the Invention
This invention relates to a mass of mica flakes which are specific in shape as well as a resin composition with the same incorporated therein. More particularly, it relates to a mass of mica flakes which can give, when incorporated in a thermoplastic resin, moldings which are excellent in mechanical strength, typically in tensile strength and bending strength, and at the same time are high in strength at weld parts and, when incorporated in a thermosetting resin, moldings which are high in strength and modulus of elasticity.
2. Description of the Prior Art
It is well known that reinforcement of plastic materials with a mica flake mass results in the production of moldings which are high in modulus of elasticity and rich in heat resistance, chemical resistance, electric insulator function and barrier function, among others. For instance, U.S. Pat. No. 3,764,456 and Canadian Pat. No. 893,163 disclose that addition of a mass of mica flakes having a high aspect ratio to a thermoplastic or thermosetting resin followed by molding gives moldings with high modulus and high strength.
However, when such a mica-filled thermoplastic resin is molded, especially by injection molding, the so-called welds may appear in the moldings. Occurrence of such welds in the molding rather leads to a decrease in strength at those parts as compared with the unreinforced resin. To avoid this drawback, attention has been paid to the designing of molds. However, careful designing of molds alone is not sufficient for the prevention of weld formation. Investigations from the resin composition viewpoint are also necessary. It is known that the strength at weld parts in a mica flake-filled thermoplastic resin depends on the diameter of mica flakes. Thus, the smaller the flake diameter, the more improved the strength at weld parts is. However, if the mica flake diameter is reduced so that the strength at weld parts might be increased, the aspect ratio necessarily becomes smaller and as a result the reinforcing effect contrarily decreases. There also arises a problem that the comminution cost for reducing the mica flake size markedly increases. A further problem is that, when the resin is a thermosetting one, incorporation of mica flakes into the thermosetting resin results in a remarkable increase in the viscosity of the resulting mixture, a decrease in the flowability, insufficient immersion, insufficient defoaming and insufficient filling, hence poor moldability, so that moldings having the desired characteristic properties cannot be obtained. As an attempt to solve these problems, there has been proposed, for instance, a method of avoiding the above troubles by admixing an unsaturated polyester in the powder form with mica flakes by the dry blend technique, and a method of preventing the troubles encounterd at the time of blending which comprises putting mica flakes in a fibrous reinforcement and immersing the resulting composite reinforcement with a resin. However, the first method, namely dry blending of an unsaturated polyester with mica flakes, is disadvantageous in that powdery polyester resins are very unique resins and are not readily available and that the powdery dry blends are difficult to mold into other forms than the flat or sheet form. Accordingly, said method is not yet in practical use. The second method, which comprises preparing a composite reinforcement beforehand, is hardly applicable to the cases where the spray up method, premix method, BMC (bulk molding compound) method or SMC (sheet molding compound) method is used although it is useful in the cases where FRP (fiber-reinforced plastic) moldings are produced by the hand lay up method, preform matched die method, cold press method, continuous panel method or the like.